Gas metal are welding (GMAW) is commonly used to join pieces of metal in high throughput production environments, particularly assembly lines employing automated or robotic welders. A challenging application of such processes is to weld relatively thin pieces of metal together, in particular aluminum or aluminum alloys. One critical issue is obtaining sufficient penetration of each workpiece by the liquid metal without creating over-penetration or “burn-through.” A second critical issue is creating a weld that can span variations in alignment gaps between workpieces, or joint “fit-up”. These issues are intertwined by the fact that higher voltage, which generally leads to welds that can span larger gaps, will in turn lead to greater burn-through. A third issue is solving these problems while simultaneously maintaining or increasing welding speed and weld mechanical properties. A fourth issue is providing a solution for the above problems that is compatible with direct current (DC) welding and alternating current (AC) welding.
Attempts have been made to solve these intertwined problems. One technique involves using alternative shielding gas compositions. GMAW, by definition, uses a gas to control the atmosphere around the weld, excluding species that react with the liquid metal. For example, a shield gas can be high purity argon (99.997% pure, <5 ppm water). Recently, small amounts of other species, for example nitrogen, oxygen, nitrous oxide, and carbon dioxide have been combined with noble gases in shielding gas compositions. The non-noble gases are used in amounts from about 200 ppm to about 1,200 ppm. These attempts have led to improvements in arc stability in direct current GMAW, but still leave issues with burn-through unresolved, particularly with respect to variations in joint fit-up.
Another attempt to solve the problem employs alternating current, and means to minimize variations in the arc current. This results in minimized burn-through but does not address the issue of tolerating variations in joint fit-up.
One attempt to solve these problems involves using a shielding gas composed of a noble gas and other reactive gases in the several hundred parts per million range (ppm).
Another technique involves using alternating current to generate the arc, and controlling the alternating current to control penetration depth. Although this reduces burn-through, field experience indicates that assembly lines incorporating these systems must be run at a slower speed.
Yet another attempt proposes a small amount of oxygen combined with a range of helium between about 10% and about 98%, with the balance being argon. However, helium and argon have substantially different effects on weld properties. For example, bead size in the resulting weld varies with helium concentration. This affects burn-through, the ability to span various joint gaps, and consequently the maximum welding speed achievable at a given weld quality level. The relationship between these quantities as a function of helium concentration is not taught by this proposal. Furthermore, the proposal teaches the use of only a limited frequency range of alternating current. Thus, this proposal does not teach an understanding of, or a solution to, the present problem.
There is therefore a need for a high-speed welding process that can weld a variety of joint gaps on relatively thin workpieces, while simultaneously minimizing burn-through and maintaining or increasing weld mechanical properties.